To enhance the performance of metal-oxide-semiconductor (MOS) devices, stress may be introduced in the channel regions of the MOS devices to improve carrier mobility. Generally, it is desirable to induce a tensile stress in the channel region of an n-type MOS (“NMOS”) device in a source-to-drain direction, and to induce a compressive stress in the channel region of a p-type MOS (“PMOS”) device in a source-to-drain direction.
A commonly used method for applying stress to the channel region of a device is growing stressor regions in the source and drain regions of the device. As an example, stressor regions in a PMOS device may comprise SiGe, while stressor regions in an NMOS device may comprise SiP. Growing stressor regions in the source and drain regions of the device can include the steps of forming a gate stack on a silicon substrate, forming spacers on sidewalls of the gate stack, forming recesses in the silicon substrate and adjacent the gate spacers, and epitaxially growing stressor regions in the recesses using an epitaxial growth process. An annealing may also be performed. In a PMOS device, since SiGe has a greater lattice constant than silicon, it expands after annealing and applies a compressive stress to the channel region of the respective MOS device, which is located between a source SiGe stressor and a drain SiGe stressor. In an NMOS device, since SiP has a smaller lattice constant than silicon, it contracts after annealing and applies a tensile stress to the channel region of the respective MOS device, which is located between a source SiP stressor and a drain SiP stressor.
Atoms and/or molecules from a precursor of the epitaxial growth process may react with gate spacers of the gate stack. Such a reaction could cause defects, e.g., selectivity loss defects, on the gate spacers that are difficult to remove by typical cleaning processes. It may also be difficult to quickly and precisely detect such selectivity loss defects. Improved methods of detecting defects caused by the epitaxial growth process may be necessary.